Pulsed lasers producing optical pulses with short temporal duration and high peak powers may be used to create laser sparks and initiate combustion. When the pulsed laser beam is focused to a small point, the intensity (power per area) at that point can be large enough to initiate electrical breakdown in the gas, thereby forming a spark (plasma). The physical mechanisms postulated for breakdown include photochemical absorption, multi-photon ionization, and electron cascade.
In an electron cascade, it is assumed that a small number of electrons appear in the beam focus region. These electrons acquire energy from the electric field by absorption of photons, and collide with neutral atoms, a process termed “inverse bremsstrahlung”. The electrons ionize the gas when their energy exceeds the ionization potential of the atoms. The electron collision will ionize the atom, producing additional electron(s) to start the cascade process and lead to avalanche breakdown.
The minimum amount of energy or intensity required to cause the breakdown is commonly referred to as the breakdown threshold. For nano second pulse durations and milli joule energy levels, breakdown is thought to be intensity limited. In addition, the breakdown threshold is also dependent on the gas composition and pressure existing in the spark target environment.
Experimental measurements of spatially and temporally averaged optical intensities are found by dividing the laser power (pulse energy/pulse duration) by the beam area. At the spark location the beam area is typically small, with dimensions on the order of 10 to 100 μm, and in many experiments it has not been precisely measured. Therefore, there tends to be some uncertainty in published intensity requirements for breakdown and spark formation. Additional uncertainty in intensity requirements is due to spatial and temporal averaging. For conditions of interest, including nano second pulse durations and milli joule energy levels with a target sparking environment comprising low-particulate (or particulate free) gas mixtures with a significant fraction of air and pressures of approximately 1 to 30 atmospheres, the required optical intensity to spark is approximately 0.5 to 10×1011 W/cm2.
For sparking uses associated with combustion engines, the desired combination of lean mixtures and high brake mean effective pressure results in the cylinder pressure and mixture density in modern engines being relatively high, creating difficulties for traditional spark ignition systems. As the density increases in the cylinder, the breakdown voltage (minimum voltage required to form a spark using a spark ignition system) also increases, ultimately to such high voltage levels that traditional spark ignition systems encounter problems with dielectric breakdown leading to unwanted sparking from the ignition leads and other undesired locations (i.e., the spark does not form between the electrodes as intended). Even if the high voltage can be managed, high voltage means that electrode erosion can be quite high. The combination of spark plug erosion and dielectric breakdown is a limiting factor in the operational envelope of modern gas engines. Optical sparks suffer from neither of these shortcomings and thus may have significant advantages for improved engine operation. In certain cases, optical sparks can also afford performance benefits associated with extension of maintenance intervals as well as changes in the lean limit, coefficient of variation of pressure, pollutant emissions, and other parameters.
Laser ignition has been shown to be a particularly effective way of igniting lean mixtures. It is fairly easy to create a spark by using “open path” laser delivery. The open path method implies that the laser beam propagates through the ambient air and is steered to the desired location by mirrors. Although simple and effective, this system is not practical for most industrial applications. Thus, there is a need for development and demonstration of a fiber optic delivery system.
The key challenges associated with the use of fiber optic delivery are the intensity damage threshold of the fiber optic material and limitations on focusing fiber optically delivered light. The former point relates to material properties of fiber material, typically silica, and limits the maximum achievable optical intensity at the fiber exit to approximately 1 to 5×109 W/cm2. Generally, the desired spark location is not right at the fiber exit, but is located some distance downstream of the fiber exit, so that intermediate optics are used to capture the light leaving the fiber and to focus it at the desired spark location. Because the intensity at the fiber exit is limited, the imaging or focusing requirements to generate a sufficient intensity to spark at the desired spark location become more stringent. In other words, the light exiting the fiber must be demagnified to enable a sufficiently high optical intensity that exceeds the breakdown threshold at the desired spark location.
The problem is compounded by the second challenge which is the difficulty in focusing fiber optically delivered light. The minimum achievable spot size (i.e. beam dimension at the focal spot) tends to increase for a laser beam that has passed through a fiber optic. This increase in minimum achievable spot size, which makes it more difficult to reach high intensity, is related to a degradation of the spatial quality of a laser beam caused by transmission through a fiber. The spatial quality of a laser beam, typically characterized by its M2 parameter, is a function of the transverse spatial modes of which the beam is composed. (A low M2 parameter corresponds to a beam composed of “lower order” spatial modes, and such modes can be focused to smaller dimensions.) Generally, the M2 parameter of the beam exiting the fiber is relatively large, and larger than the value for the beam entering the fiber. The spatial quality (and M2) of light exiting a fiber is influenced by the fiber diameter and the exit angle of light leaving the fiber. For small-diameter single-mode fibers (diameter<˜30 μm) the degradation is minimal; however, such fibers cannot transmit a large amount of energy and are not considered useful in laser ignition application(s). Larger diameter fibers are required to transmit higher energies, but in such cases the larger diameter increases the beam degradation and thus impedes focusing to small spot sizes (high intensities).
Solid core fiber optics have one optical material in the core (center channel) and a second optical material in the cladding (surrounding material). The index-of-refraction of the core material is selected to be larger than that of the cladding material so that light at the core-cladding interface is “totally internally reflected” and thus guided through the fiber core. Hollow core fibers have a hollow bore (no material) surrounded by a wall material. Such a configuration has a higher index in the wall than the core and does not allow efficient light guiding. Uncoated hollow fibers may only be effectively used in straight geometries.
Finally, it noted that it is much more difficult to form a spark in the gas phase as compared to on a solid or in a liquid because more optical intensity is required. There are a number of papers/approaches that form sparks off solid surfaces after fiber delivery, and this can be done rather “routinely” with a solid fiber. For the same reason, it is also routine to spark in gases containing dust, sprays, or particulate matter since the spark initially forms on those liquids/solids as opposed to in the gas. However, it is desirable to spark in the gas phase because it allows the spark to be located away from cylinder walls or other solid surfaces, which act as heat sinks and yield poorer combustion performance. Freedom in locating the spark may also allow sparking at other locations that offer other combustion benefits (for example, locations where the air/fuel mixing is better or the gas velocity field is favorable).
Another consequence of the ease of sparking on solids is that the use of fiber optics becomes harder because of the tendency to spark (unwantedly) at the launch entrance of the fiber. Such sparks consume energy from the laser beam and may degrade the quality of the beam preventing subsequent sparking after the fiber